In its monthly report, the cartel said demand for crude was expected to reach 88.14 million barrels per day (mbd) in 2011, down from a previous estimate of 88.18.
The new figure represented a demand rise of 1.2 mbd from 2010.
For 2012, OPEC now forecasts a demand of 89.44 mbd, down from a previous forecast of 89.50 mbd.
"Economic worries along with high oil prices have affected OECD oil demand, leading to weaker-than-expected consumption during the summer driving season," said the group, whose members provide about 35 percent of the world's crude oil and has more than three-quarters of its reserves.
"Oil demand in the OECD is expected to continue its contraction after a temporary rebound last year."
The situation in the United States, which saw its "AAA" credit rating downgraded on Friday by Standard & Poor, was cited as a particular concern.
"Should the situation see further deterioration in the US, then aggregate oil demand will see a further decline this year," said OPEC of the world's biggest oil consumer.
Copyright AFP (Agence France-Presse), 2011